The capability of the insurgents emerged in more than eight hours of unedited training videos shot at a jihadi 'weapons lab' in the IS stronghold of Raqqa.

Sir Michael said of the footage obtained by Sky News: "It does appear to be an intelligence gold mine.

Video:IS May Have Greater Scientific Base

"The thing that particularly got my attention was the possibility that they would be able to get their hands on old surface-to-air missiles and insert this thermal battery which would make it very concerning indeed.

"And there are an awful lot of these old missiles around the place, of course.

"These would not necessarily be of great use in Iraq and Syria but in wider Europe and one's immediate concern would be towards civilian airliners.

Video:'Jihadi University' Builds Missiles

"A civilian airliner coming into land say at Heathrow, Gatwick or Stansted, anywhere on the continent, in America, in Russia is a very, very easy target for a surface-to-air missile of this sort.

"It's an infrared missile, they will pick up the heat from the aircraft's engines, it will lock itself on and the chances are it will hit."

Speaking to Sky News, Paul Wolfowitz, the former US deputy secretary of defence, said: "The point in your report that I think is most concerning is the possibility that they know how to activate old shoulder-fired surface-to-air missiles of which unfortunately there are a very large number.

Video:Inside IS Weapons Lab

"If the batteries don't work they are not such a threat. If the batteries do work they are a very grave threat to civilian airliners at low altitudes."

Mr Wolfowitz said of the footage: "It underscores what we should have been sensitive to already which is these people are very very dangerous, they have widespread networks and they have a lot of technical expertise.

"A major effort needs to be made, not to ultimately to defeat ISIS, but to defeat ISIS on a schedule that's as rapid as possible and as decisive as possible - and that's not going to be easy to do now that it's morphed to the extent that it has and grown to the extent that it has."